1. Field of the Invention
This invention relates to a tibial element for a replacement knee prosthesis. More particularly, the invention relates to a modular tibial element in which the stem may be placed in a predetermined position with respect to a separate tibial tray.
2. Description of the Prior Art
Tibial elements for replacement knee prostheses are known which are provided with one or more selectable alternative stems which can be selected by the surgeon to meet the requirements of the patient. In devices of this kind, the selected stem is usually threaded into or onto the tibial tray and due to the stems themselves usually being relatively circularly symmetrical about a longitudinal axis, their angular position relative to the tibial tray is immaterial. For example, see U.S. Pat. No. 4,822,366. There are, however, advantages in being able to provide stems which are non-symmetrical, but in order to do this it is necessary to be able to locate them in relation to the tibial tray so that the desired relative configuration is obtained. The present invention is intended to provide a construction to allow this.
U.S. Pat. No. 4,936,853 relates to a modular knee prosthesis in which a selected stem is located into a tibial tray by means of a self-locking morse taper. A number of alternative modular stems are described which may be provided with flutes or have radially projecting flanges. Such flanges are shown in U.S. Pat. Nos. 4,822,362 and 4,938,769.
In constructions of the type set forth above, the surgeon selects the appropriate stem for the patient concerned at the time of the operation and the stem is then assembled to the tray in the surgery. Difficulties arise, however, during assembly due to the tendency for the stem to rotate in the tapered socket in the tray when the locking device, usually a pin or bolt, is tightened. It is often necessary for there to be accurate alignment between the cross-sectional shape of the stem and the configuration of the tray, and this tends to be lost during tightening. This misalignment is particularly acute if the lower portion of the tray has a shaped engagement feature intended for engagement with the condylar area of the tibia of the user. The present invention is intended to overcome the disadvantage set out above.
U.S. Pat. Nos. 4,578,081 and 4,790,854 disclose one method of aligning a morse taper stem connection, but not in relation to a tibial tray.